1. Field of the Invention
The invention is concerned with heat exchangers for hot flowing gases, especially gases resulting from the thermic splitting or cracking of gaseous and liquid hydrocarbons, and particularly with heat exchangers which may be connected in parallel, between a splitting furnace and a collector wherein oil may subsequently be injected into the gases. More particularly, this invention relates to heat exchangers having a plurality of individual heat exchanger devices, each individual heat exchanger having a pipe-shaped passageway for hot gas, each passageway being disposed within a cooling jacket along a portion of its length, and wherein each heat exchanger device is comprised of concentric pipes which are connected with one another and are impermeable to gas.
2. Description of the Prior Art
Heat exchangers of the type to which the invention relates are used in the cooling of process gases, especially gases resulting from the thermic splitting of gaseous and liquid hydrocarbons. Such heat exchangers are designed as single pipe apparatus which respectively attach to individual furnace splitting pipe exits and, at their inlets, have a cross-section which corresponds to that of the exits of the splitting furnace. In splitting furnaces which employ single-stage cracked gas cooling, in designing the heat exchanger, the band width of the raw materials to be used (gas to gasoil) must be taken into consideration. As a rule, the cracked gas exit temperature from the heat exchanger is chosen such that, in the clean condition of the apparatus, an energy recuperation which is still economical is attained when using light charging stock. When heavy stock is employed, the build-up of deposits on the cooling surfaces of the heat exchanger, i.e., coking, is still acceptable with such a chosen heat exchanger exhaust gas temperature. In general, the temperature range of the cracked gas downstream of the heat exchanger is limited to 420.degree.-550.degree. C. with a clean heating surface. An oil injector will typically be located downstream of the heat exchanger, the heat exchanger generally being a single-stage device, and the injector will produce a further cooling of the cracked gas.
The heat exchanger cooling surfaces, in time, become very dirty as a result of the build-up of deposits, i.e., due to coking, with the use of heavy stock and a gas exit temperature of 650.degree. C. may be reached downstream of the heat exchanger. It is, accordingly, necessary to periodically de-coke the heat exchanger.
In splitting furnaces which employ two-stage cracked gas cooling, the heat exchanger system has customarily comprised of a large number of single-stage exchangers, which are connected to individual furnace exits. The single-stage heat exchangers discharge into a collector at a temperature of about 550.degree.-650.degree. C. When charging with light stock, i.e., gas or gasoline, an additional cooling of the cracked gases is accomplished in a further heat exchanger which, as a rule, is a large volume single apparatus for each furnace unit.
In the case of such furnaces wherein two-stage cooling is provided and heavy stock is used, i.e., gasoil hydrated residue, the cracked gas is taken directly out of the collector downstream of the first heat exchanger stage and is subjected to oil-injection. Thus, when heavy stock is used the additional heat exchanger is not employed.
The disadvantage of single-stage cooling as described immediately above is that the splitting process is limited to use of a relatively narrow range of feed stock if heavy coking is to be avoided. On the other hand, if one compromises here, then with light stock the energy gain is less than is technically possible. If necessary, a limitation of the gas travel time through the cooling system can ensue with coking during heavy stock charging. The advantages of single-stage heat exchangers are that they are relatively uncomplicated and reasonably priced, do not require hot connecting pipes between two serially connected heat exchanger stages and are characterized by an uncomplicated method of construction.
Previously available heat exchanger systems employing two-stage heat exchanger devices have been characterized by very high cost, because of the division of the first and second heat exchanger stages into two separate systems, which division demands a very elaborate hot interconnection, i.e., pipes which operate at a temperature in the range of 650.degree.-680.degree. C. In addition, the pressure losses of the entire system are high because of the relatively long pipes. Such pressure losses, in turn, result in the loss of splitting in the splitting furnace. An important advantage of two-stage heat exchanger systems is the possibility of flexibility since the operation can be adapted to suit the charging stock, i.e., only single-stage heat exchanger may be employed for heavy and two-stage heat exchangers may be employed for light stock.